Various types of magnetic field sensing elements are known, including Hall Effect elements and magnetoresistance elements. Magnetic field sensors generally include one or more magnetic field sensing elements along with other electronic components. Some magnetic field sensors also include a fixed permanent magnet.
Magnetic field sensors provide an electrical signal representative of a sensed magnetic field. In some embodiments, the magnetic field sensor provides information about a sensed ferromagnetic object by sensing fluctuations of the magnetic field associated with a magnet part of the magnetic field sensor as an object moves within a magnetic field generated by the magnet. In the presence of a moving ferromagnetic object, the magnetic field signal sensed by the magnetic field sensor varies in accordance with a shape or profile of the moving ferromagnetic object.
In other embodiments, the magnetic field sensor has no magnet, and the magnetic field sensor provides information about position of a sensed object to which a magnet is coupled.
Magnetic field sensors are often used to detect movement of features of a ferromagnetic gear, such as gear teeth and/or gear slots. A magnetic field sensor in this application is commonly referred to as a “gear tooth” sensor.
In some arrangements, the gear is placed upon a target object, for example, a camshaft in an engine, thus, it is the rotation of the target object (e.g., camshaft) that is sensed by detection of the moving features of the gear. Gear tooth sensors are used, for example, in automotive applications to provide information to an engine control processor for ignition timing control, fuel management, and other operations.
In other embodiments, a ring magnet with a plurality of alternating poles, which can be ferromagnetic or otherwise magnetic, is coupled to the target object. In these embodiments, the magnetic field sensor senses rotation of the ring magnet and the target object to which it is coupled.
Information provided by the gear tooth sensor to the engine control processor can include, but is not limited to, an absolute angle of rotation of a target object (e.g., a camshaft) as it rotates, a speed of rotation, and, in some embodiments, a direction of rotation. With this information, the engine control processor can adjust the timing of firing of the ignition system and the timing of fuel injection by the fuel injection system.
When used in automotive applications, the power supply for the magnetic field sensor originates with an automobile battery. The battery supplies a nominal voltage of twelve volts, but the battery voltage is subject to very large voltage swings during operation of an automobile. For example, when an automobile engine is undergoing starting, due to a high current draw of an electric starter motor, the battery voltage can experience a large drop in voltage, down to a minimum of about four volts. Shortly thereafter, when the high current draw abruptly stops, voltage on wiring in the automobile, and/or the battery voltage, can experience a very high voltage, i.e., a voltage transient, above fifty volts, for example, 100 volts, due to inductance in the starter motor, in the battery, and in the automobile wiring.
Magnetic field sensors used in automobiles, and in some other applications as well, must both survive the high voltage and also be able to operate at the minimum battery voltage, e.g., four volts.
Magnetic field sensing elements used in magnetic field sensors tend to have sensitivities that are directly related to a voltage with which they are powered or driven. Thus, the minimum battery voltage of about four volts greatly limits the sensitivity of the magnetic field sensing elements within magnetic field sensors.
It would be desirable to provide a magnetic field sensor that can operate with low voltages supplied to the magnetic field sensor, but which can still provide high sensitivity, higher than that which is associated with the low voltages.